The damage done by nuclear radiation depends on how much of which type of radiation you were exposed to how often.

Reuters: Kim Kyung Hoon

Atomic weapons and nuclear accidents like those at Chernobyl and Fukushima have made sure we all know that nuclear radiation can kill.

But how exactly does the radiation affect our bodies? And why does radiation sometimes cause cancer, and other times cure it?

Nuclear radiation is the energy given off by all radioactive elements when they break down into more stable atoms. And it is being produced in and around you right now.

Radioactive atoms in everything — from rocks to bananas and even our bodies — give off energy as they decay to more stable forms.

Our cells can easily clean up any damage done by this low-level background radiation — on average Australians are exposed to about 1.5 millisievert (mSv) of background radiation a year — but it is a different story if you are exposed to doses greater than about 500 mSv.

The combination of how much radiation you are exposed to, what type, and how often will determine the effect on your cells and tissues.

Low doses of nuclear radiation are more likely to change cells by modifying DNA, while high doses tend to kill cells.

So long-term exposure to low doses of radiation increase the odds of getting cancer, while a single high dose will quickly cause immediate damage to cells and tissues — a process used effectively to kill tumour cells in radiation therapy.

Very high doses like those experienced by workers at the site of nuclear accidents (several thousand times higher than the background radiation level) cause extensive damage, resulting in a range of symptoms known collectively as radiation sickness. Extremely high doses can kill in days or weeks.

Health effects of ionising radiation

Dose range

Effects on human health (including the unborn child)

Up to 10 mSv

No direct evidence of human health effects

10 - 1000 mSv

No early effects; increased incidence of certain cancers in exposed populations at higher doses

What is nuclear radiation?

The high-energy radiation given off by radioactive decay can take the form of very high speed particles (electrons in the case of beta radiation; two protons and two neutrons in alpha radiation) or waves (gamma or X-rays).

Regardless of the form it takes, all nuclear radiation has enough energy to strip electrons off atoms and molecules that it interacts with, earning it the name ionising radiation.

It is this electron-stripping (ionising) property that does the damage to our cells and tissues.

As well as generating heat, the removal of electrons can break chemical bonds. When that happens in a molecule of DNA it can cause mutations, which can lead to cancer down the track. And ionising a protein can mess with its shape and function — not something you want in the molecules that coordinate most of the chemistry in our cells.

Those effects are compounded when water molecules (H2O) in our bodies are ionised into the high energy free radicals OH- and H+, which can go on to attack other nearby molecules and cells.

Our bodies are full of water, and almost all cells have DNA, but some cells and tissues are more susceptible to damage from nuclear radiation than others.

Which cells in the body are most affected by radiation?

The cells and organs that are most affected by nuclear radiation are the ones that are actively reproducing, because the DNA is more exposed when the cell is in the process of dividing.

Blood cells have the highest turnover rate in our bodies, so the tissue where they are produced — the rapidly dividing cells of the bone marrow — is the most susceptible to radiation damage.

What is radiation sickness?

What is radiation sickness?

Radiation sickness occurs when a person is exposed to a high dose of ionising radiation.

The severity of the symptoms and illness depends upon the type and amount of radiation, length of exposure and the part of the body exposed.

Initial symptoms include nausea, vomiting, headache and diarrhoea. These symptoms can start within minutes or days after the exposure.

People who have been exposed to high doses can also have skin damage ranging from itching to burns, blisters and ulcers. They may also have temporary hair loss.

After the initial symptoms there may be a brief period of improvement. This is followed by severe symptoms that can last days or months depending upon the dose and time exposed.

Treatment for acute radiation sickness may include bone marrow transplants, antibiotics for infections and burns management.

The damage to bone marrow in high doses — and complete destruction of it in very high doses — impairs our immune system by not replacing our white blood cells.

Long-term exposure to lower doses can lead to cancerous DNA mutations in the marrow, which can lead to the blood cancer leukaemia in people exposed through work or location.

The cells lining the digestive system are also fast-dividing, so they can cope with the physical and chemical assault of digesting our food. Gastrointestinal damage contributes to the symptoms of acute radiation syndrome in people who are exposed to high doses.

Developing foetuses are, of course, incredibly susceptible to radiation, while slow-dividing tissues like muscle and nerve cells are far less sensitive.

And healthy tissues and organs are not the only cells that regularly reproduce — tumours are literally balls of cells that are dividing out of control, which is why radiation therapy can be effective in destroying them. The good blood supply feeding tumours helps too, because the radiation interacts with the dissolved oxygen in the blood as well. That leads to the production of free radicals which attack the nearby cells, amplifying the radiation's effect.

Exposure to external radiation is one thing, but ingesting radioactive particles takes the damage to another level.

What happens if you breathe in radioactive particles or swallow contaminated food or water?

Inhaling or swallowing radioactive material delivers the source of radiation directly to your cells, increasing the risk of cancer developing in the tissues where they accumulate.

Radioactive iodine (iodine-131) blown into the atmosphere by the 1986 Chernobyl explosion caused a large number of cases of thyroid cancer in people who drank contaminated milk. (Having been released in the clouds of radioactive material following the explosion, the iodine — a by-product of nuclear fission reactions — landed on fields where it was swallowed by cows).

Iodine is essential for the normal function of the thyroid gland, and with its knack for attracting iodine the gland gets a concentrated dose of iodine-131 when contaminated milk is drunk. Thankfully, thyroid cancer is treatable by removal of the gland, although a lifetime of hormone supplements follows. With a half-life of just eight days, the level of radioactive iodine fell off quickly after the accident, so the risk of exposure dropped within weeks of the disaster.

Not so with the radioactive isotope of caesium-137, which has a half-life of 30 years. Caesium is very soluble in water, so when it enters our bloodstream via contaminated food or water it ends up spreading throughout our bodies, and concentrating in muscle tissue in particular. Our bodies eventually turn over these tissues, but it takes three months to reduce the amount of caesium in our muscles by half, so the long-term exposure to beta and gamma radiation increases the chances of cancer developing in those tissues.

With a half-life of 29 years, strontium-90 joins caesium-137 as a long-lasting source of harmful radiation after nuclear accidents.

Strontium is chemically very similar to calcium, so if you ingest food contaminated with radioactive strontium isotopes like strontium-90, it ends up wherever calcium normally would — primarily in the bones.

In adults, strontium accumulates mainly on the surface of bones, but in children it can be incorporated into the growing bone itself. The beta radiation given off as the radioactive atoms decay into more stable forms can damage the bone marrow and lead to bone cancer.